From light emitting diodes, solar cells, sensors, transistors to many other semiconductor devices, organic materials with controllable electronic and opto-electronic properties are emerging as potential competitors to silicon, gallium aresenide and other inorganic semiconductor materials as the backbones of the semiconductor industry [J. M. Shaw, P. F. Seidler, IBM Journal of Research & Development, 45(1), 3(2001)].
A simple organic semiconductor device may consist of one layer of electro-opto active organic materials sandwiched between two electrodes. However in practice, many layers of organic semiconductor materials with different energy levels and functionalities are often required in order to improve the device performance. One typical example is an organic light-emitting device (OLED) [C. W. Tang, and S. A. Vanslyke, Applied Physics Letter, 51,913(1987)], as shown in FIG. 1. On a glass substrate (1), a layer of ITO (2) is first deposited. This ITO layer (2) will act as an anode. Then, a layer of hole-transport material (4) is applied onto the anode (2). Following a layer of organic semiconductor (5) is deposited onto the layer of hole-transport materials (4), a low work function material is deposited in a vacuum chamber by thermal evaporation or sputtering to form the cathode layer (6). Finally, a protective top layer (7) is applied in order to prevent oxygen or water molecules from reaching the low work function cathode layer (6). This protective top layer (7) may be a single layer of metal, glass, or multi-layers of metals and dense polymer. A power supply may now be connected to allow a current to flow into the organic semiconductor (5) through the ITO layer (2). The flow of the current leads to recombination of charge carriers in the organic semiconductor (5) to result in the emission of light (8). In this typical OLED device (9), layer (1) is the substrate, layer (2) is the anode, layer (4) is the hole-transport media, layer (5) is the light emitting organic materials, layer (6) is the cathode and layer (7) is the protective layer. In addition, other layers, such as a layer of hole-injection materials and/or a layer of electron-blocking materials may also be inserted between anode layer (2) and hole-transport layer (4), and/or a layer of electron-injection materials and/or a layer of electron-transport materials may be inserted between cathode layer (6) and light emitting organic semiconductor layer (5). These layers are chosen to have properties such as hole or electron transport, hole or electron blockage and light emission. Hence, it is clear that these devices, including (9) are multi-layer structured.
Currently, multi-layer structured organic devices such as (9) are conventionally constructed in a sequential manner. For instance in the case of polymeric light emitting diode (9), as shown schematically in FIG. 2. Firstly, a transparent electrode (2), usually indium-doped tin oxide (ITO) is first vacuum sputtered on a glass substrate (1). Secondly a hole-transport layer (4) such as poly (3,4-ethylene-dioxythiophene) (PEDOT) is coated onto the layer (2). Thirdly, a layer of light emitting polymer (5) such as poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is coating onto the layer (4), fourthly a top electrode (6) such as barium is thermally evaporated on layer (5) through a shadow mask. Finally, a protective layer (7) such as aluminium is deposited. The above fabrication steps yield a standard polymer light-emitting device with a layer structure of Glass/ITO/PEDOT/MEH-PPV/Ba/Al. The polymer (5) is commonly applied by spin coating or ink jet printing, while the electrodes (2, 6) are usually constructed by vacuum deposition or sputtering. Therefore, in this sequential fabrication process, both wet processes and dry processes are often required. One major drawback of this sequential fabrication is the requirements to integrate the wet processes and the dry processes, with total different working environments, into a single fabrication chamber. Even though various layers on top of each other can be processed by wet processing method, the choice of solvent still remains a problem, because the solvent used for one layer may attack the previously coated layer. Another drawback of the sequential fabrication of the successive layers to form the organic devices is the compatibility of the materials, specifically the ones for anode and cathode. Moreover, organic semiconductor devices, particularly these based on conjugated polymers, are amenable to a roll-to-roll process to minimize production costs. This sequential method is not flexible enough to meet the requirements for a roll-to-roll process, especially for the production of larger size flat devices. It is thus clear that a simple manufacturing process is highly valuable for the fabrication of these multi-layer organic semiconductor devices.
In the present invention, organic semiconductor devices containing two parts, each part formed on a substrate with different functional layers, are disclosed. By fabricating specific layers on each substrate before combining the two parts to form the organic semiconductor devices or circuits, the difficulties described above on the integration of wet and dry processes can be overcome and the fabrication cost can be reduced. The disclosed structure will allow the fabrication of each of the two parts to be standardized in a working environment or equipment which has been optimized individually for each part. These two parts can be then assembled in a specific way when needed. Hence, it is clear that the disclosed structure formed by a combinational approach will dissect a big project to various building blocks. This combinational approach will reduce the equipment requirements for the fabrication. More importantly, as compared to the sequential approach, this combinational method will ultimately provide the flexibility of varying combination possibilities of the final device. For example, if five different first parts and five different second parts are produced, up to twenty-five different devices configurations can be constructed. From the above description, it is evident that it is advantageous if an organic device can be constructed by combining two different parts for mass production.